class CollectionItem2 { }

class BaseCollection2<TItem extends CollectionItem2> {
    _itemsByKey: { [key: string]: TItem; };
    constructor() {
        this._itemsByKey = {};
    }
}

class DataView2 extends BaseCollection2<CollectionItem2> {
    fillItems(item: CollectionItem2) {
        this._itemsByKey['dummy'] = item;
    }
}